1. Field of the Invention
The present invention relates to a switching mode power converter and more specifically relates to a flyback power converter.
2. Description of the Related Art
Flyback power converters are widely used in home appliances, battery chargers, and many other products. Considerable ongoing research is focused on making flyback power converters smaller, cheaper and even more efficient. A flyback power converter typically includes a PWM controller, a power MOSFET, a transformer, and a feedback-control circuit. The purpose of the feedback-control circuit is to sense the output voltage and/or the output current of the secondary side of the power supply, and to connect a feedback signal to the PWM controller through an isolated device such as an optical-coupler.
FIG. 1 shows a traditional flyback power converter. Although the circuit is able to regulate the output voltage and the output current, it has several drawbacks. One drawback of this circuit is that the size of the flyback power converter cannot be easily reduced. It is difficult to reduce the size of the flyback power converter without eliminating the optical-coupler and the secondary feedback-control circuit. Another drawback of this flyback power converter is high power consumption. To maintain a constant current output from the secondary side, the circuit includes a current-sense resistor. However, the current-sense resistor increases the power consumption of the power converter.
In recent years, several primary-side control schemes for flyback power converters have been proposed. These prior-art primary-side control schemes have attempted in various ways to reduce the size and the cost of flyback power converters. One prior-art primary-side control scheme is “Switching power supply packages” by Arthur J. Collmeyer, Mark D. Telefus, Dickson T. Wong, and David B. Manner (U.S. Pat. No. 6,434,021.) Although the circuit is able to regulate the output voltage and the output current, it has several drawbacks.
One drawback of this prior-art invention is that the pulse train generator and the pulse rate controller vary the switching frequency in response to the load, which is unacceptable for some electronic appliances. Another drawback is that the feedback control voltage is sensed from a high voltage source. This method results in a loss of accuracy, and it increases the cost of the controller. Finally, the voltage drop of the output rectifier is not compensated for. Thus, the output voltage of this prior-art invention will deviate significantly from a constant DC level.
Another prior-art control scheme is “Method and apparatus providing a multi-function terminal for a power supply controller” by Balu Balakrishnan, Alex B. Djenguerian, and Leif O. Lund (U.S. Pat. No. 6,538,908.) The drawback of this prior art is that the optical-coupler and the secondary feedback circuit for loop control are still required. Otherwise, the output voltage and the output current will fluctuate significantly.
Reflected voltage control has also been proposed as a means for primary-side control. Two prior-art patents teaching this method include “Switched mode power supply responsive to voltage across energy transfer element” by Balu Balakrishnan, David Michael, and Hugh Matthews (U.S. Pat. No. 6,233,161) and “Switched mode power supply responsive to current derived from voltage across energy transfer element input” by Balu Balakrishnan, David Michael, Hugh Matthews (U.S. Pat. No. 6,480,399.)
One principal drawback of these two prior arts is inaccurate feedback control. In order to generate a feedback control signal, the reflected voltage of the transformer is filtered and turned into a DC voltage and/or current through a resistor-capacitor circuit. However, this reflected voltage includes not only the output voltage information, but also the spike voltage generated from the leakage inductance of the transformer. Thus, the output voltage of this prior-art invention will deviate significantly from a constant DC level. Furthermore, the voltage drop of the output rectifier is not compensated for in the feedback loop. When load changes occur, this problem will introduce additional distortion into the output voltage.
Another drawback of these two prior-art inventions is high power consumption. The reflected voltage is filtered to supply power for PWM control. However, the resistor of the filter burns the majority of the reflected power, even if PWM control only consumes a little power. Therefore the power consumption of the power supply is high.
Thus, a need still remains for an efficient primary-side flyback power converter with a well regulated, constant output voltage and output current.